I'm trying to identify several screws. Guides online are fairly straightforward - measure the diameter of the thread part, and pitch if possible. However, none of my screw measurements make any sense. I've ruled out my caliper as the cause.
Here's some of my measurements (Suspected screw: Expected -> Actual):
- Known M2.5: 2.5mm -> 2.43 mm
- M3: 3mm -> 3.38 mm, 0.5mm pitch
- M4: 4mm -> 3.88 mm
- Known 5/16": 0.3125" -> 0.305", 18 threads/inch
My measurements seem to be off my 2% to 10%. Am I missing something? Is my M3 screw actually M3.5 or M4? I can't find, say, 3.38mm on any screw charts (example)